Whoever has said that you cannot use some specific technology because of ISO27001 probably has some other vested interest - possibly even just laziness. The standard does not mandate what you can and cannot do. It effectively defines the way you need to manage risk and how you need to demonstrate that you are managing risks.
There are many organisations who are compliant and who use Office 365 or even Google Work. Even more that have all manner of external communications from Skype to Slack, etc.
What matters is having a robust process for identifying and acknowledging the risks and mitigating them.
A specific example may help. Though not directly related to ISO27001, there are a number of doctors in the UK who are holding patient consultations over Skype. That requires certified risk management well beyond that normally expected from ISO27001 (think the UK equivalent of the US's HIPPA).